The present invention relates to a method of determining the measuring uncertainty of a coordinate measuring machine.
Coordinate measuring machines--hereinafter referred to simply as CMMs--are machines for checking the dimensions of parts of all shapes, and are of enormous assistance in computer-aided production techniques (from CAD/CAM to CIM) of numerically controlled machining centers.
CMMs operate on the principle of sampling the surface of a part and, by means of a tracer movable along three perpendicular Cartesian axes, acquiring a predetermined number of surface points by which to reconstruct the shape of the part by means of an interpolation algorithm.
By virtue of their high degree of measuring precision, CMMs are used to advantage for checking machining tolerances, as opposed to less accurate conventional measuring instruments, but supply a numeric result, with no indication as to the uncertainty, of the measurement.
This therefore poses the problem of determining the uncertainty of CMM measurements to determine the measurand values defining the ambiguous regions, i.e. the regions in which it is difficult to establish whether or not the part conforms to specifications. The ambiguous regions, in fact, define both sides of the tolerance region, i.e. the range defined by the minimum and maximum values the measured quantity may assume to be considered valid. If the measurement lies in an ambiguous region, the part is normally considered outside the tolerance range and rejected, hence the obvious advantages of reducing the ambiguous regions by reducing the uncertainty of the measurement.
ISO 9000 quality standards require that the measuring uncertainty of test and measuring equipment be known and compatible with measuring requirements.